The Phase Structure And Electrochemical Properties Of Non-AB₅ Type La-Ni Based Alloys

In this thesis, previous research work on the phase structure and hydrogen storage properties of the non-AB5 type La-Ni based hydrogen storage alloys have been comprehensively reviewed. On this basis, some of the non-ABj type La-Ni based alloys, including LaNij , Jba-jNi? > LaNi2 binary alloys and LaM^Mx and La2Ni7-TMy(M=Al, Nfau Co) ternary alloys, were chosen as objects of study. By means of XRD analysis and electrochemical measurements, the phase structure and electrochemical properties of above alloys were investigated. The effects of melt-spining and anneal treatment on the phase structure and electrochemical properties of some alloys were also studied. The purpose of such investigation is to get some basic data and understanding about the electrochemical properties of non-ABs type La-Ni based hydrogen storage alloys.For as-cast LaNi3 INi?, LaNi2 and LaNi4 alloys, multi-phase structures were observed in all of these four alloys. The maximum discharge capacities of these alloys obtained were: LaNi4(275.7mAh/g), La2Ni7(248.7mAh/g), LaNi3(199.4mAh/g) and LaNi2(137.9mAh/g) respectively, which are much lower man these theoretical capacity. Among these four alloys, LaNi3 (HRD6oo=70.9%) has the best high-rate discharge ability while LaNLi (HRD6oo=63.8%) has the worst. It could also be seen that from the capacity retention rate after 70 cycles, LaNi2 (S7o=64.5%) has the best cycling stability while LaNu (870=26.2%) has the worst The melt-spun LaNi3 and La2Ni? alloys have almost the same phase structures with corresponding as-cast alloys, however, the amount of various phases changed to some extent Annealing treatment of LaNis alloys under various circumstance results in different phase structures. In the case of 700 "C X 120h, annealed LaNi3 has LaNi3 as main phase, while in the case of 900 "C X 120h, the main phase of annealed alloy was I^Ni?. The amount of main phase in annealed alloy also increased compared to as-cast and melt-spun alloys. Compared to as-cast alloy, annealing treatment lowered the maximum discharge capacity but improved the high-rate discharge ability and cycling stability.The phase structure and electrochemical properties of AB3-type LaNi3_xMx (M=A1, Co, Mn) alloys have been investigated For the LaNi3.xAlx(x=0.00~0.45), the main phase changed from LaNi3 to- fc^Ni? when x>0.20 , but for in the LaNi3.yCoy (y=0.0~0.6) and LaNi3.zMnz (z=0.0~0.6) alloys, the main phase maintained LaNi3 in spite of the increase of Co or Mn amount The discharge capacity of LaNi3.xAlx decrease from 188.8mAh/g to 216.7mAh/g. Compared to LaNi3 alloy, partial substitution of Ni by Al improved dramatically the high-rate discharge ability (HKD6oo=78.7%~91.4%) and cycling stability, which could be derived from the feet that 870 increased from 55.7% to 73.1% with the increase of Al substitution. The maximum discharge capacities (153.2-203.6mAh/g) of LaNi3-yCOy alloysdecreased to some extent, while the high-rate discharge abilities were improved. Partial substitution of Ni by Ma lowered the maximum discharge capacities (160.2~202.3mAh/g). It was found that among the aforesaid ternary La-Ni based alloys, LaNi28Mu has the maximum discharge capacity (216.7mAh/g) and better cycling stability (870=66.0% after 70 cycles).For LaNi3-xAlx (x=O.200.45), LaNi3-yCOy (y=0.1-0.6) and LaNi3.zMnz (z=0.1-0,6) AB3-type afloys, annealing treatment (700*C X 96b.) resulted in increase of the amount of the main phase and the amount of other phases decreased correspondingly. Compared to as-cast alloys, the discharge capacities of annealed LaNi3.xAlx (x=0.20~0.45) alloys (199.9~237.3mAh/g) were improved to a certain extent while the high-rate discharge abilities (HRD6oo=70.6%~88.5%) were lowered. In the case of LaNi^COy (y=0.1-0.6) alloys, the activation properties improved appreciably and the cycling stabilities (S7o=67.3%~78.8%) improved remarked, while the maximum discharge capacities (163.0~179.8mAh/g) were lowered after annealing treatment In the case of LaNi3.zMn2 (z=0.1-0.6) alloys, annealing treatment resulted in deterioration of...